Re: How Often Does a Detailed Level of Setting Material Matter?
The amount of detail you need will also depend largely with the setting you're using. I'm more into sci-fi, so I'll use examples from that corner of the multiverse...
Say you're running a Babylon 5 game. Over the course of the series, we got a pretty good idea who the various alien races were, what their general behavior patterns were, how their societies functioned, etc...with a few exceptions. But if you decide you're going to run a B5 game, there are pretty limited options for introducing aliens of a species unknown to anyone, most of reachable space has been claimed by one race or another, and there are clear-cut expectations of how different groups should be treated and how they'll react to that treatment. You, as GM, have to be familiar with that to maintain the feel of a B5 story...drift too far out of those norms and it starts to become just another generic sci-fi setting.
Now, say, you're doing Star Wars, instead. Star Wars seems to have an inexhaustible supply of new aliens. Every movie that comes out mixes some new species in with the more familiar faces. Same holds true for planets. There are certain fixed points in the canon that should be portrayed accurately, if the game takes you there...but there's an awful lot of space in a galaxy that can let you experiment with all kinds of aliens, societies, organizations, etc etc. As long as it's generally 'Star Wars flavored', you have a lot of leeway.
Or say you go even more obscure, and try something with Space: Above and Beyond (if you remember the TV show...it got the Fox Firefly treatment back when Joss Whedon was still dreaming up Firefly...) The only planet that has any real definite conditions to be aware of is Earth...and, since we're already there, that's not too challenging. It needs to be a little more futuristic, but the series is full of tongue-in-cheek references to pop culture of the 20th Century, so you can have some fun. The planets? To the best of my knowledge, there's only a handful of them even mentioned by name, even fewer that appear on any sort of map in the show, and a scant few that are supposed to be in any sort of proximity to each other. We are given precious few actual facts about the Chigs, a few more about the Silicates...and that's about it. Interstellar travel is possible, but they cheat the hows of it, and while the Chigs are the only aliens that Humanity has faced as of when the TV show happened, that doesn't mean that you can't introduce more (although I'd keep them pretty scant.) But you've got a very loose frame in which to operate, there's a lot of wiggle-room and not a huge amount of source material, which almost makes it incumbent upon the GM to start making stuff up to fill in the gaps.
The flipside is, the more popular your setting is, the more likely you are to get players...but the fewer options there are to makeup up your own stuff within that setting (with some exceptions...Star Wars, I already mentioned, and Star Trek are both examples of the setting actually gaining more options as it expanded, instead of having more stuff nailed down, because they're always introducing new places, new species and races, new technologies, etc, so you can either run with the tried-and-tested canon materials or start building onto the galaxy as you see fit.)